Has anyone got much experience with them? My only experience is with common properties that just have the offer price rubber stamped.

However, with more unusual properties such as small holdings or one off properties with land, they seem to be rather random and frankly meaningless.

I'm aware that they aren't a general "this property will eventually sell for x" type thing, but rather "the lender could shift the repossessed property for Y at auction" but even so the numbers don't seem to add up.

many years ago i mended houses for folk ,my impression of mortgage valuations was that it very much depended on the person valuing it rather than the real or even potential value ,some would reduce by 20% for a single rotten windowsill and some would ignore obvious dry rot or subsidence entirely.

i expect it is even more random /subjective with unusual properties it certainly was with gd 1 and 2 listed stuff.

The thing about one off properties is that they're, well, one off, meaning it's saleable for a fairly random amount, depending on nothing more than a couple of people's finances. The cost of land and a rebuild cost is easy enough to estimate, but beyond that, property prices are fairly random for that kind of property, and so impossible to value on paper, really.

As for regular boxes, zoopla is sufficient, on average, which is how the game is played.

A lot of mortgage companies just do a quick check on Zoopla nowadays, I'm told

That was my thought, either Zoopla was used or whatever Zoopla bases it's valuations on. The problem with that is the property in question has not been sold for decades so Zoopla doesn't have anything to go on - not even the number of rooms - so it's estimate has little meaning.